The present invention relates to a thermal transfer image-receiving sheet. More particularly, the present invention is concerned with a thermal transfer image-receiving sheet capable of forming a record image excellent in coloring density, sharpness and various types of fastness, particularly durability such as light fastness, fingerprint resistance and plasticizer resistance.
Various thermal transfer printing processes are known in the art. One of them is a transfer printing process which comprises supporting a sublimable dye as a recording agent on a substrate sheet, such as a polyester film, to form a thermal transfer sheet and forming various full color images on an image-receiving sheet dyeable with a sublimable dye, for example, an image-receiving sheet comprising paper, a plastic film or the like and, formed thereon, a dye-receiving layer.
In this case, a thermal head of a printer is used as heating means, and a number of color dots of three or four colors are transferred to the image-receiving material by heating for a very short period of time, thereby reproducing a full color image of an original by means of the multicolor dots.
Since the color material used is a dye, the image thus formed is very clear and highly transparent, so that the resultant image is excellent in reproducibility and gradation of intermediate colors, and according to this method, the quality of the image is the same as that of an image formed by conventional offset printing and gravure printing, and it is possible to form an image having a high quality comparable to a full color photographic image.
Not only the construction of the thermal transfer sheet but also the construction of an image-receiving sheet for forming an image are important for usefully practicing the above-described thermal transfer process.
For example, Japanese Patent Laid-Open Publication Nos. 169370/1982, 207250/1982 and 25793/1985 disclose prior art techniques applicable to the above-described thermal transfer image-receiving sheet, wherein the dye-receiving layer is formed by using a polyester resin, vinyl resins such as a polyvinyl chloride, a polycarbonate resin, a polyvinyl butyral resin, an acrylic resin, a cellulose resin, an olefin resin and a polystyrene resin.
In the above-described thermal transfer image-receiving sheet, it is known that the dyeability of the dye-receiving layer and various types of durability and storage stability of an image formed thereon greatly vary depending upon the kind of the resin constituting the dye-receiving layer.
The dyeing capability of the dye which is transferred can be improved by improving the diffusivity of the dye at the time of the thermal transfer through the formation of the dye-receiving layer from a resin having a good dyeability or the incorporation of a plasticizer in the dye-receiving layer. In the dye-receiving layer comprising the above-described resin having a good dyeability, the formed image blurs during storage. Therefore, the storage stability is poor or the fixability of the dye is poor, so that the dye bleeds out on the surface of the image-receiving sheet, which causes other articles in contact with the surface of the sheet to be liable to staining.
The above-described problems of storage stability and staining can be solved by selecting such a resin that the dye transferred to the dye-receiving layer is less liable to migration within the dye-receiving layer. In this case, however, the dyeing property of the dye is so poor that it is impossible to form an image having a high density and a high sharpness.
There are other large problems such as the light fastness of transferred dye, fading of the formed image due to sweat or sebum migrated to the image surface when a hand touches the image portion, swelling or cracking of the image-receiving layer per se, fingerprint resistance, migration of the dye when the dye contacts with a substance containing a plasticizer, such as an eraser or a soft vinyl chloride resin, that is, plasticizer resistance.
Three-dimensional crosslinking of the resin layer for receiving a dye is considered as means for solving the above-described problems, and several proposals have been made on the three-dimensional crosslinking. Examples thereof include a method disclosed in Japanese Patent Laid-Open Nos. 215398/1983, 199997/1986, 34392/1990, 178089/1990 and 86494/1990 wherein the three-dimensional crosslinking is conducted by reacting a polyester resin with a polyisocyanate and a method disclosed in Japanese Patent Laid-Open Nos. 160681/1989, 123794/1989 and 126587/1991 wherein the three-dimensional crosslinking is conducted by reacting a vinyl chloride/vinyl acetate copolymer having active hydrogen with a polyisocyanate.
In these methods, however, the amount of an active hydrogen having an isocyanate group, which can be introduced into one molecule, is limited. For example, in the case of a polyester resin, although the proportion of the hydroxyl group can be increased by reducing the molecular weight, the proportion of the hydroxyl group is necessarily low when the polyester resin has a commonly used molecular weight (a number average molecular weight of 10,000 or more). Further, in the case of a vinyl chloride/vinyl chloride acetate copolymer resin, although a hydroxyl group can be introduced by saponifying vinyl acetate monomer units, an increase in the proportion of the hydroxyl group (40% by mole or more) causes the copolymer to become insoluble in a general purpose solvent other than an alcohol. Therefore, the amount of introduction of the hydroxyl group should be reduced, which causes the crosslinking density of the dye-receiving layer to be low.